The present invention relates to satellite based telecommunications systems. More specifically, the invention relates to an autonomous communications channel assignment system.
Today, satellite systems have been proposed to transmit and receive telecommunications signals to and from user terminals. Such satellite based telecommunications systems may utilize a constellation of satellites and at least one Network Operations Center (NOC) to relay communications signals to and from the user terminals (fixed or mobile). The satellites may be cross-linked. Each satellite includes at least one antenna which defines the satellite's coverage region on the earth called its footprint. The satellite antenna(s) may divide the footprint into multiple beam spots called cells, or the footprint may be a single cell. Each cell is assigned one or more frequency bandwidths (subbands), along which communications signals travel between the satellite and each user terminal within that cell. Each of these subbands may support communications from a plurality of user terminals. In addition, adjacent cells may not be able to use the same subbands simultaneously. Depending on the system design, there may be a minimum distance between cells that may re-use the same subbands simultaneously. This distance is referred to as the frequency reuse distance.
The user terminals communicate along preassigned communications channels having a preassigned subband centered about a carrier frequency. In a frequency division multiple access system (FDMA), only one user terminal within a cell may transmit at a particular carrier frequency or slot. In a time division multiple access system (TDMA), multiple user terminals in one cell may transmit on one particular carrier frequency. In this latter type of system, each user terminal is assigned one or more time intervals or slots during which it may communicate over the shared carrier frequency. An FDM/TDMA system uses multiple frequencies and time slots. Multiple user terminals in a given cell, or within the frequency reuse distance, communicate with a single satellite or adjacent satellites in a non-interfering manner by using different carrier frequencies during the same time slot, and/or by using the same carrier frequency (frequency slot) but only during mutually exclusive time slots.
As illustrated in FIG. 3, each user terminal communicates by transmitting data packets at a preassigned frequency slot during a preassigned time slot to the satellite that defines the cell within which the user terminal is located. In order to obtain an initial time and frequency slot to use for communication, the user terminal sends an access request to the NOC, informing the NOC that the user terminal desires a channel within the system (step 36). The access request is conveyed to the NOC via one or more satellites (step 38). In response thereto, the NOC selects an available time and frequency slot. The NOC responds by sending the selected time and frequency slot assignment to the new user terminal, again via one or more satellites (steps 40 and 42). Once the user terminal receives the assigned time and frequency slot, the user terminal tunes its frequency and timing accordingly and may begin communicating (step 44).
The user terminal transmits data packets of information which include a header containing the destination address to which the data should be transmitted (step 46). Throughout the time during which a user terminal communicates over the assigned time and frequency slots, the NOC checks to see if any contentions may occur for time and frequency slots currently in use (steps 48 and 50). The necessary operations conducted by the NOC can be implemented in a computer program which can be partitioned into subroutines for each satellite coverage area. The NOC identifies contentions by conducting a current satellite orbit submodel (SOS). The current SOS represents a subset or submodel of a model of the system's overall satellite orbit constellation (hereinafter, the satellite orbit model or SOM). The SOS further comprises a plurality of SOS assignments which correspond to each user terminal's time and frequency slot assignments at a given time (starting point). A contention may be due to either predictable or unpredictable events.
An example of a common predictable event, is when a user terminal must switch to a different cell due to known (e.g., predictable) movement of the satellite relative to the Earth. For example, if a particular satellite contains Chicago within its coverage area, and that satellite is orbiting the Earth at a known speed, the system can predict the exact moment at which that satellite will move into a position containing, for example, Los Angeles in its coverage area and out of the location where it covers Chicago. Because the NOC keeps track of the precise location and speed of every satellite by running the system's overall SOM, the NOC knows when to perform a user terminal call handoff.
Another predictable event, for example, relates to the situation in which a given satellite footprint may contain a non-uniform distribution of users within different cells of the footprint. As the satellite moves and the cells move across the earth, the quantity of users in the various cells changes with some cells increasing quantity of users and other cells decreasing quantity of users. Call handoffs for some users in the cells with increasing quantity of users may be needed to avoid contention. These required handoffs can be predicted by the SOM and by the individual SOSs.
Unpredictable events include occurrences such as when (1) a current user terminal wants to increase the data transmission rate; (2) a current user terminal must switch to a different cell because of the user terminal's movement; and (3) several new user terminals request access to the system, thereby increasing the system's overall congestion sufficiently to make the current SOS assigned by the NOC inefficient. In some systems, predictable events typically occur more frequently than unpredictable events. Each time a predictable or unpredictable event occurs that creates a contention, the NOC must assign new values for the user terminal's time and/or frequency slots (step 52). The NOC assigns, to the user terminal, new time/frequency slots by transmitting the new assignment information over a communications channel associated with the user terminal. The NOC is able to determine what time/frequency slot values are available because it keeps track of every channel being used in the system by continuously running the system's overall SOM. Thus, based on the available channels, the NOC may assign new time/frequency slots to any user terminal. This assignment information is considered overhead communications as it uses up bandwidth which would otherwise be available for carrying communications data. Hence, the transmission of assignment information decreases the overall capacity of the system. At times the amount of assignment information becomes quite significant, and thus has a substantial detrimental impact upon the bandwidth available to carry communications data.
A need remains for improved capacity for satellite-based telecommunication's systems.